In packet-based multimedia networks, such as those in accordance with the International Telecommunications Union (ITU) Recommendation H.323 or the Internet Engineering Task Force (IETF) RFC 3261 (SIP), the capability to initiate a call without completing it, delay alerting the called user or entity, or prematurely releasing the call before alerting the called user or entity, may prove useful. This deliberate delay in alerting the user, or prematurely releasing the call, is referred to here as delayed call establishment.
Generally, two types of calls are used in H.323 networks. The first is real, or actual, calls, where one endpoint calls the other and the user's phone rings. The other type of call may be referred to as non-call associated services, such as a message waiting indicator. Within SIP networks, only real or actual calls exist.
In the case of real calls, it is impossible for a system to initiate a call for whatever purpose for which it does not want the user to be aware, as it would cause the user's phone to ring. On the other hand, if the call were initiated for some purpose and the goal is to be able to complete the call only after certain events occur or certain requirements are satisfied, albeit with a delay, non-real calls are not useful, as they cannot be converted into real calls.
The reasons for initiating a call, but not completing it or alerting the called party, may be varied. For example, a calling endpoint may want to perform a feature discovery process on the other endpoint. If the features between endpoints match, the call could then be progressed to the established state. However, in current implementations, the user's phone would ring and then there would be a delay while feature discovery is performed. The possibility exists that features will not match and the call cannot be completed.
Similarly, the ability to delay call establishment may be useful for providing services such as call completion on busy and call completion on no answer, where for instance, an intermediate device between the calling endpoint and the receiving endpoint continues to call the receiving endpoint until the line is not busy. Once the line is no longer busy, the intermediate device calls the calling endpoint and completes the call. Currently, this is not possible in H.323 or SIP networks. Other uses may include quality of service testing and coordinating with policy enforcement functions to manipulate media ports during call establishment.